Urban Heat Island | UPSC

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Urban Heat Island (UHIs): Causes, Effects and Mitigation 

  • Urban heat islands (UHIs) are urbanised areas that experience higher temperatures relative to their surrounding areas.  
  • Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies.
  • Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. These pockets of heat are referred to as “urban heat islands” or simply “heat islands.”
  • Heat islands can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season.

Urban Heat Island

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Causes of UHIs

  • Heat islands form as a result of several factors:

a) Reduced Natural Landscapes in Urban Areas

    • Trees, vegetation, and water bodies tend to cool the air by providing shade, transpiring water from plant leaves, and evaporating surface water, respectively.
    • Hard, dry surfaces in urban areas – such as roofs, sidewalks, roads, buildings, and parking lots – provide less shade and moisture than natural landscapes and therefore contribute to higher temperatures.

b) Urban Material Properties

    • Conventional human-made materials used in urban environments such as pavements or roofing tend to reflect less solar energy, and absorb and emit more of the sun’s heat compared to trees, vegetation, and other natural surfaces.
    • Often, heat islands build throughout the day and become more pronounced after sunset due to the slow release of heat from urban materials.

c) Urban Geometry

    • In heavily developed areas, surfaces and structures obstructed by neighbouring buildings become large thermal masses that cannot release their heat readily.
    • Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects.

d) Heat Generated from Human Activities

    • Vehicles, air-conditioning units, buildings, and industrial facilities all emit heat into the urban environment. These sources of human-generated, or anthropogenic, waste heat can contribute to heat island effects.

e) Weather and Geography

    • Calm and clear weather conditions result in more severe heat islands by maximising the amount of solar energy reaching urban surfaces and minimising the amount of heat that can be carried away.
          • Conversely, strong winds and cloud cover suppress heat island formation. 
    • Geographic features can also impact the heat island effect. For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city.
  • Differences in night-time temperatures between UHIs and surrounding rural areas are usually more than differences in daytime temperatures.
        • Humid regions and cities with larger and denser populations experience the greatest temperature differences.
  • This is because concrete, which is used widely in cities, has a very high heat capacity and acts as a reservoir of heat.
  • In addition, atmospheric conditions above cities often lead to urban air being trapped near the ground surface, where it is heated by the warm urban surfaces.
  • Furthermore, the tall canyons formed by city buildings block the loss of heat from ground surfaces through radiation. This is called the urban canyon effect.

Surface Heat Islands Vs Atmospheric Heat Islands

  • In general, temperatures are different at the surface of the earth and in the atmospheric air, higher above the city. For this reason, there are two types of heat islands: surface heat islands and atmospheric heat islands.

a) Surface Heat Islands

  • These heat islands form because urban surfaces such as roadways and rooftops absorb and emit heat to a greater extent than most natural surfaces.
  • Surface heat islands tend to be most intense during the day when the sun is shining.

b) Atmospheric Heat Islands

  • These heat islands form as a result of warmer air in urban areas compared to cooler air in outlying areas.
  • Atmospheric heat islands vary much less in intensity than surface heat islands. UHIs Effects
  • Elevated temperatures from heat islands can affect a community’s environment and quality of life in multiple ways.

a) Increased Energy Consumption

  • Heat islands increase demand for air conditioning to cool buildings. Heat islands increase both overall electricity demand, as well as peak energy demand.

b) Elevated Emissions of Air Pollutants and Greenhouse Gases

  • Heat islands raise demand for electricity in summer. Companies that supply electricity typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas emissions.
  • In addition to their impact on energy-related emissions, elevated temperatures can directly increase the rate of ground-level ozone formation. 
        • Ground-level ozone is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight and hot weather.
        • If all other variables are equal, more ground-level ozone will form as the environment becomes sunnier and hotter.

c) Compromised Human Health and Comfort

  • Heat islands contribute to higher daytime temperatures, reduced nighttime cooling, and higher air-pollution levels.
        • These, in turn, contribute to heat-related deaths and heat-related illnesses such as general discomfort, respiratory difficulties, heat cramps, heat exhaustion, and non-fatal heat stroke.
  • Heat islands can also exacerbate the impact of naturally occurring heat waves, which are periods of abnormally hot, and often humid, weather.

d) Impaired Water Quality

  • High temperatures of pavement and rooftop surfaces can heat up stormwater runoff, which drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes.
  • Water temperature affects all aspects of aquatic life, especially the metabolism and reproduction of many aquatic species.
  • NASA’s Ecosystem Spaceborne Thermal Radiometer Experiment (Ecostress) recently pointed out heat islands in urban parts of Delhi, where temperatures were far higher than nearby agricultural lands.
  • Ecostress is an instrument with a radiometer which was sent onto the International Space Station in 2018. It can measure temperatures on the ground, as opposed to the air temperature.

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